A drilling fluid used in the rotary drilling of subterranean wells is expected to perform many functions. For example, the drilling fluid needs to carry cuttings from beneath the drill bit up the annulus, thereby allowing their separation at the surface. At the same time, the drilling fluid is also expected to cool and clean the drill bit, reduce friction between the drill string and the sides of the hole, and maintain stability in the bore hole's uncased sections. The drilling fluid is also expected to form a filter that seals openings in the formations penetrated by the bit so as to reduce the unwanted influx of formation fluids from permeable rocks. In addition, in drilling subterranean wells, formation solids often become dispersed in the drilling fluid. These formation solids typically comprise the cuttings produced by the drill bit's action and the solids produced by the bore hole's instability. The presence of either type of formation solids in the drilling fluid can greatly increase drilling time and costs, especially if the formation solids are clay minerals that swell. The overall increase in bulk volume accompanying clay swelling impedes removal of cuttings from beneath the drill bit, increases friction between the drill string and the sides of the bore hole, inhibits formation of the thin filter that seals formations, and causes loss of circulation or stuck pipe. Accordingly, another function of the drilling fluid is to reduce the adverse effects of formation solids, particularly clay minerals that swell.
The clay minerals that are encountered in the drilling of subterranean wells are generally crystalline in nature, with a flaky, mica-type structure. The “flakes” of the clay are made up of a number of crystal platelets stacked face-to-face. Each platelet is called a unit layer, and the surfaces of the unit layer are called basal surfaces. A unit layer is composed of multiple sheets. One type of sheet, the octahedral sheet, is composed of either aluminum or magnesium atoms octahedrally coordinated with the oxygen atoms of hydroxyls. Another type of sheet, the tetrahedral sheet, consists of silicon atoms tetrahedrally coordinated with oxygen atoms. Sheets within a unit layer link together by sharing oxygen atoms. When this linking occurs between one octahedral and one tetrahedral sheet, one basal surface consists of exposed oxygen atoms while the other basal surface has exposed hydroxyls. Alternatively, two tetrahedral sheets may bond with one octahedral sheet by sharing oxygen atoms. The resulting structure, known as the Hoffman structure, has an octahedral sheet that is sandwiched between the two tetrahedral sheets. As a result, both basal surfaces in a Hoffman structure are composed of exposed oxygen atoms. The individual unit layers of the clay are stacked together face-to-face, and are held in place by weak attractive forces. The distance between corresponding planes in adjacent unit layers is called the c-spacing.
In clay mineral crystals, atoms having different valences commonly will be positioned within the sheets of the structure to create a negative potential at the crystal surface. When the clay crystal is suspended in water, a cation may be adsorbed on the surface, and these absorbed cations, often called exchangeable cations, may chemically trade places with other cations. In addition, ions may also be adsorbed on the clay crystal edges and exchange with other ions in the water.
The type of substitutions occurring within the clay crystal structure and the exchangeable cations adsorbed on the crystal surface greatly affect clay swelling. Clay swelling is a phenomenon in which water molecules surround a clay crystal structure and position themselves to increase the structure's c-spacing, which causes an increase in the volume of the clay. Two types of swelling may occur, either surface hydration or osmotic. Only certain clays, such as sodium montmorillonite, exhibit osmotic swelling, whereas all clays exhibit surface hydration swelling.
Surface hydration swelling involves the hydrogen bonding of water molecules to the oxygen atoms exposed on the crystal surface, which results in layers of water molecules aligning to form a quasi-crystalline structure between the unit, thereby increasing the c-spacing. In osmotic swelling, if the concentration of cations between unit layers in a clay mineral is higher than the cation concentration in the surrounding water, water will be osmotically drawn between the unit layers, thereby increasing the c-spacing. Osmotic swelling typically causes the clay to swell more than surface hydration.
Exchangeable cations found in clay minerals are reported to have a significant impact on the amount of swelling that takes place. The exchangeable cations compete with water molecules for the available reactive sites in the clay structure. Generally, cations with high valences are more strongly adsorbed than cations with low valences. Thus, clays with low valence exchangeable cations will swell more than clays whose exchangeable cations have high valences.
In the North Sea and the United States Gulf Coast, drillers commonly encounter argillaceous sediments in which the predominant clay mineral is sodium montmorillonite (commonly called “gumbo clay”). Sodium cations are predominately the exchangeable cations in gumbo clay. Because the sodium cation has a low positive valence (+1 valence) it easily disperses into water. Consequently, gumbo clay is notorious for its swelling. Thus, given the frequency in which gumbo clay is encountered in drilling subterranean wells, the development of a substance and method for reducing clay swelling is of primary importance in the drilling industry.
One commonly employed method to reduce clay swelling is the addition of salts to the drilling fluids. However, salts flocculate the clays, which causes both high fluid losses and an almost complete loss of thixotropy. Further, increasing salinity often decreases the functional characteristics of drilling fluid.
Accordingly, there is a long felt need for a drilling fluid additive that acts to control clay swelling in drilled formations without adversely effecting the properties of drilling fluids; a drilling fluid that contains such drilling fluid additive; and a method of reducing clay swelling in a drilled formation. The present invention is directed towards meeting these needs.